1. Field of the Invention
The present invention relates to apparatus for discriminating between light emitted from sources having respectively different ratios of brightest to dimmest intensities during a given time period. Such apparatus is particularly useful in cameras wherein one or more corrective filters, which compensate for differences in color temperature of light sources, may be moved into and out of the optical path to provide correct color balance and exposure. Alternatively, the film may be exposed and encoded so that compensation may be effected during printing.
2. Description of the Prior Art
It is well known that color film which is sensitized to be balanced for indoor tungsten illumination must be used with a color compensating filter if it is to be used outdoors in natural sunlight. Of course, film sensitized for natural light would need a filter when used indoors. In prior art cameras havin automatic color filter control, electrical means have been provided for measuring the spectral characteristics of scene illumination and for automatically moving filters into and out of the optical path to correct the spectral transfer characteristics of the camera lens systems, depending upon the scene illumination characteristics and the spectral characteristics of the film.
In determining whether the scene is illuminated by natural or artificial light, some prior art devices measure the amount of infrared components in the scene light, as for example in U.S. Pat. No. 3,651,749 entitled "Exposure Control Apparatus" and which issued to G. H. DeGroot on Mar. 28, 1972. Other known cameras, operating on the same "spectral characteristic" principle, have means for comparing the relative amounts of red and blue light from the scene. By assuming that when the scene light is predominantely red, the photograph is being taken indoors and that when the scene light is mostly blue, the scene is outdoors, the camera can automatically adjust its filtering system in accordance with the perceived illumination source. U.S. Pat. No. 3,475,616 entitled "Automatic Color Filter Control" and which issued on Oct. 28, 1969 to A. G. Stimson et al discloses such a system.
While such devices normally work well for their intended purposes, they can be "fooled" as, for instance, when there is an unusual amount or lack of infrared light in a naturally or artificially lighted scene, or when a scene is dominated by single colors which might be used by the camera mechanism to indicate the light source.
Further, the prior art devices do not readily enable discrimination between different sources of artificial light, such as for example between fluorescent and tungsten lamp sources.